A methanol carbonylation process (methanol-acetic acid process) is known as a process for industrially producing acetic acid. With this process, an acetic acid product is produced typically by allowing methanol to react with carbon monoxide in the presence of a catalyst in a reactor, to form acetic acid in a reaction mixture, evaporating the reaction mixture using an evaporator into a vapor phase, and purifying the vapor phase through a light ends column, and subsequently through a dehydration column to give an acetic acid product. Alternatively, the product from the dehydration column is further fed to a subsequent heavy ends column and, in some cases, a subsequent product column to give an acetic acid product.
In such an acetic acid production process, formic acid is produced as a by-product in the reactor. The minimum amount of formic acid is favorable because the formic acid reduces the purity of an acetic acid product. Patent Literature 1 and 2 each disclose that formic acid is formed through the reaction of carbon monoxide with water; and therefore, the formic acid concentration in acetic acid product can be lowered by controlling a water concentration in a reaction medium to a low level. However, there is the problem that a catalyst tends to become unstable if the water concentration in the reaction medium is decreased.